1. Field of the Invention
The present invention relates in general to a control system for a machine tool and more particularly, to a control system for a numerically controlled machine tool having control circuit arrangements which control the machine tool in response to a signal from a touch sensitive tool received in a machine tool spindle for centering the axis of the spindle on the center of a workpiece bore as well as for measuring the diameter of the workpiece bore.
2. Description of the Prior Art
Recently, there has been suggested a numerically controlled machine tool which is capable of automatically aligning the axis of a tool spindle with the center of a preliminary bore of a workpiece in advance of a finish boring. In a centering operation of the machine tool, a touch sensitive tool is inserted into the tool spindle in the same manner as the case of a boring tool, and the tool spindle is moved along two orthogonal controlled axes in the workpiece preliminary bore. A distance along each controlled axis (between two positions at each of which the touch sensitive tool received in the tool spindle contacts the internal surface of the preliminary bore) is calculated to detect the center of the preliminary bore, onto which the axis of the tool spindle is then positioned. Further, in the machine tool, it is possible to measure the diameter of the workpiece bore if modifications are made to move the tool spindle from the centering position, along one of the controlled axis, until the touch sensitive tool contacts the workpiece bore and then calculate the feed amount through which the tool spindle is moved during such transfer.
In order for the machine tool to perform precise spindle centering and precise diameter measuring, the touch sensitive tool must be received in the tool spindle with its axis exactly aligned with the rotational axis of the tool spindle. However, the exact alignment of the sensing tool axis with the spindle rotational axis cannot practically be realized because of either or both of: misalignment which the machine tool inherently has between the spindle rotational axis and the axis of the tool receiving bore; and foreign matters such as chips which, together with the touch sensitive tool, enter into the tool receiving bore. For these reasons, it is difficult to perform precise spindle centering and precise diameter measuring in the machine tool.